Thermal hydraulic study on detection of random failure of fuel by delayed neutron detection system

During a fuel element failure in a liquid metal cooled fast breeder reactor, the fission products originating from the failed pins mix into the sodium pool. Delayed neutron detectors are provided in the sodium pool to detect such failures by way of detection of delayed neutrons emitted by delayed neutron precursors present in the fission products. The transient evolution of precursor concentration is governed by the sodium flow distribution in the pool. Transient thermal hydraulic analysis of precursor concentration evolution in the hot pool of a 500 MWe fast reactor has been carried out using the computational fluid dynamics code PHOENICS to estimate the time at which the failures of various fuel subassembly are detected. Standard k–ɛ turbulence model is used to take care of turbulence in conjunction with the precursor concentration equation. It has been found that in order to effectively detect the failure of all fuel SA in the core, a minimum of eight detectors are essential to be provided in the hot pool.